Well drilling bits provided with such a passageway are particularly useful for performing operations in the wellbore ahead of the drilling bit quickly after drilling operation has stopped, without the need to first retrieve the drilling bit to the surface. Such operations can for example include formation testing (logging), or drilling of a pilot hole of smaller size.
U.S. Pat. No. 5,244,050 discloses a well drilling bit comprising a bit body provided at its face with one or more fixedly attached roller cones. The bit body is attachable to a tubular drill string, and is internally provided with a passageway providing fluid communication between the interior of the attached drill string and the exterior of the bit body. The passageway opens towards the exterior of the bit body through a port in the face of the bit body, which port is arranged in a region where no roller cone is attached. The bit body comprises a hinged closure means for selectively closing the port. When the port is open, a tool such as a logging tool or a pilot drill string can be passed from inside the drill string through the passageway into the well exterior of the well drilling bit. In conventional roller-cone bits without a passageway, the roller cones cover virtually all the face of the bit body, and shape and relative arrangement of the cones and the cutters on the cones are designed for optimum drilling performance. A well-known roller-cone bit that is widely used in the art is known as the tricone bit wherein three substantially equal cones carrying teeth or cutters are arranged symmetrically on the bit face. The size and geometry of the cones, the arrangement of the teeth or cutters on the cones, the precise alignment, bearing and materials used are optimised in conventional roller cone bits depending on the particular application.
In contrast, in comparison with conventional roller-cone bits, at least one of the roller cones has to be left out in the well drilling bit according to the USA patent, in order to allow sufficient space for a port. This well drilling bit therefore has the disadvantage that in order to provide the passageway the drilling performance is decreased in comparison with that of a conventional roller-cone bit.